The propagation of energy through a medium is often described as the propagation of a wave through the medium. If the medium is not a vacuum, the medium will cause some degree of distortion to the wave. For example, if the medium is the atmosphere, small local variations in air density cause time varying effects that are broadly classified as scintillation. Scintillation reduces the acuity of imagery and reduces the ability to focus laser beams that have propagated through the atmosphere. These effects are well documented in the art and tend to be most severe near the earth's surface where air density and air heating, due to the proximity with the earth's surface, are both greatest. For example, if you look far down a road on a very hot and sunny day, you will often see what is usually called a mirage. What you are seeing is the rapidly changing temperature in the air causing it to act like a thick, constantly bending lens. These effects have always degraded astronomical observations. Consequently, observatories tend to be built on high ground where the air is less dense and also look up (to space) at steep angles to reduce the path through the atmosphere.
In recent years, technology has advanced to allow active control systems that monitor the atmospheric distortion and then compensate for the measured distortion. These active systems are referred to as adaptive optics (AO). The basic idea of an adaptive optics system is to rapidly sense the wavefront errors and then to correct for them on timescales faster than those at which the atmosphere changes. Consequently, there are really three parts to an adaptive optics system: 1) a component which senses wavefront errors/distortions; 2) a control system which figures out how to correct these errors/distortions; and 3) an optical element which receives the signals from the control system and implements wavefront corrections (e.g., a deformable mirror controlled by the control system). The device that senses the distortions in the incoming wavefront of light is called a wavefront sensor.
The most commonly used approach for a wavefront sensor is the Shack-Hartmann method. As shown in FIG. 1, this approach is completely geometric in nature and so has no dependence on the coherence of the sensed optical beam. The incoming wavefront is broken into an array of spatial samples, called subapertures of the primary aperture, by a two dimensional array of lenslets. The subaperture sampled by each lenslet is brought to a focus at a known distance F behind each array. The lateral position of the focal spot depends on the local tilt of the incoming wavefront; a measurement of all the subaperture spot positions is therefore a measure of the gradient of the incoming wavefront. A two-dimensional integration process called reconstruction can then be used to estimate the shape of the original wavefront, and from there the distortion is determined so that the correction signals for the deformable mirror can be derived. Thus, large amount of computation is necessary just to provide the estimated shape of the original wavefront in order to determine the distortion.
An AO system with such type of wavefront sensor is practical under benign conditions that produce wavefront distortions that change at relatively slow rates. However, high speed changes of wavefront distortions that occur in atmospheric paths near the earth's surface are more difficult to correct because they require intensive computations to determine each correction. Implementing such intensive computations is not always practical when imaging targets and propagating laser beams through atmospheric paths near the earth's surface.
The inventors have recently implemented a new technique that uses a holographic optical element (HOE) to dramatically reduce the calculations needed to determine wavefront distortions. The HOE divides the distortion into a number of orthogonal components. The HOE is designed to focus the energy corresponding to a particular component along a predetermined line. The location of the focused spot along the line is designed to indicate the intensity of the component. The measurement of the locations of the spots, corresponding to the intensity of the various orthogonal components, provides a measurement of the wavefront distortion without the need for intensive computer computation. Since intensive computations are not required, this technique appears to be practical for determining wavefront distortions that occur in atmospheric paths near the earth's surface.
With this in mind, the inventors initially implemented this technique using a 2-dimensional charged couple device (CCD) to read out the position of each spot. However, it was soon discovered that the needed update rate far exceeded the maximum readout rate of the CCD. Subsequently, the inventors implemented this technique using a set of 1-dimensional CCDs. Each linear CCD was used to read out the orthogonal components of the distortion. The linear CCDs were operated in parallel to greatly increase the update rate. However, even this increase in update rate was not sufficient to measure the high frequency components of the wavefront distortion. In addition, it was found that the actual distortion magnitude often fell between the calibration points. For example, if the calibration points of the particular orthogonal component of the distortion included a ½ wave and a ¾ wave and that actual distortion was 0.6 wave, two spots were generated, one at ½ wave and a less intense spot at ¾ wave. The CCD would read out both spots, but then a computation based on the relative intensity at the two locations was required to determine the actual magnitude of the distortion. Such required computation slowed the process so that this implementation also was not practical for use in imaging targets and propagating laser beams through atmospheric paths near the earth's surface.
Thus, none of the inventors' prior attempts to implement using a holographic optical element (HOE) to dramatically reduce the calculations needed to determine wavefront distortions has proven satisfactory.
However, because an HOE dramatically reduces the calculations needed to determine wavefront distortions, making it practical for use where rapid determination of wavefront distortions is necessary (e.g., wavefront distortions that occur in atmospheric paths near the earth's surface), there is a need to provide a way of reading out the distortion information of an HOE that is faster than using CCDs.